How to Check an ABN or ACN Number (Australia)
In Australia, businesses quote an ABN (Australian Business Number) on invoices, and companies also have an ACN (Australian Company Number). Checking them catches typos before an invoice goes out — and missing or invalid ABNs can trigger withholding at the top tax rate.
ABN: 11 digits
An ABN looks like 51 824 753 556. Its validity is checked by a weighted sum:
- Subtract 1 from the first digit.
- Multiply each digit by its weight: 10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19.
- Add the results. A valid ABN's sum is divisible by 89.
ACN: 9 digits
An ACN such as 004 085 616 uses weights 8 to 1 on the first eight digits; the remainder of the sum divided by 10, subtracted from 10, gives the last digit (10 becomes 0). A company's ABN often contains its ACN: the ACN plus two leading digits.
Checking instantly
Paste numbers into the ABN and ACN validator — one, or a whole list with one per line. It detects which kind each number is, runs the check digits and formats the result. Everything runs in your browser.
What a valid check doesn't prove
The check digits only show the number is well formed. To confirm it's active, who it belongs to and whether the business is registered for GST, use the government's free ABN Lookup. Do this for new suppliers before paying.
Adding GST to a quote? Australia's rate is 10% — the VAT / GST calculator adds or removes it.